1. Field of the Invention
The invention relates to a power supply apparatus for supplying an operating voltage to a microcomputer.
2. Description of Related Art
Conventionally, an electronic control unit (ECU) mounted on an automobile or a vehicle uses a microcomputer to execute various processes for providing control. In recent years, microcomputers used for onboard ECUs are increasingly requested to operate faster in accordance with the increase in sophistication of the control requirements. Accordingly, the increased performance requirements for a microcomputer of an ECU necessitate that an internal core operate at a voltage, such as 1.2 V, that is lower than an input/output (I/O) port or an I/O circuit that interchanges signals with external circuits.
A power supply apparatus for supplying an operating voltage to the microcomputer is provided with a function of outputting a reset signal to the microcomputer so as to prevent a malfunction due to a decreased operating voltage supplied to the core of the microcomputer.
FIG. 4 shows an example of a conventional type of power supply apparatus. A power supply apparatus 100 in FIG. 4 is provided for an onboard ECU and supplies an operating voltage to a microcomputer 1 in the ECU. A battery voltage is supplied via an ignition switch or a relay, though not shown. The power supply apparatus 100 supplies the battery voltage as a power supply voltage V1 from the outside and steps down the voltage V1 to two types of constant voltages V3 and V4 for output. The voltage V3, 1.2 V in the present example, is supplied as an operating voltage to a core 2 in the microcomputer 1. The voltage V4, 5 V in the present example, is supplied as an operating voltage to an I/O port 3 in the microcomputer 1. The voltage V4 is also supplied to circuits other than the microcomputer 1 in the onboard ECU as needed. Such circuits include, for example, an input circuit for receiving a sensor signal from the outside and a drive circuit for driving an external electric load.
The power supply apparatus 100 includes a smoothing circuit 11, a switching regulator 19, a series regulator 23, a series regulator 27, a capacitor C3, a capacitor C4, and a reset control circuit 30. The smoothing circuit 11 is supplied with the power supply voltage V1 from the outside. The switching regulator 19 steps down the power supply voltage V1 to an intermediate voltage V2 such as 6 V higher than the voltage V3 or V4. The series regulator 23 steps down the intermediate voltage V2 output from the switching regulator 19 to the voltage V3. The series regulator 27 steps down the intermediate voltage V2 to the voltage V4. The capacitor C3 stabilizes the voltage V3 output from the series regulator 23. The capacitor C4 stabilizes the voltage V4 output from the series regulator 27. For example, 3P-2004-153931 A describes such a power supply apparatus including series-connected switching regulator and series regulator.
The smoothing circuit 11 includes a low-pass filter including a choke coil L1 and a capacitor C1. The switching regulator 19 includes a switching transistor 13 or metal oxide semiconductor field effect transistor (MOSFET) in the present example, a switching regulator control circuit 15, and a smoothing circuit 17. The smoothing circuit 17 includes a free wheeling diode D1, a choke coil L2, and a capacitor C2.
The series regulator 23 includes a transistor 21 for output control and a series regulator control circuit 22. Similarly, the series regulator 27 includes a transistor 25 for output control and a series regulator control circuit 26.
In the power supply apparatus 100, the smoothing circuit 11 eliminates noise components higher than a specified frequency from the power supply voltage V1. The power supply voltage V1 is then applied to the switching transistor 13 of the switching regulator 19.
The switching transistor 13 turns on or off in accordance with a control signal from the switching regulator control circuit 15. The switching transistor 13 outputs a pulse-shaped voltage. The smoothing circuit 17 transforms the pulse-shaped voltage into an almost stabilized average voltage. The switching regulator control circuit 15 monitors the voltage V2 smoothed by the smoothing circuit 17, namely the output voltage from the switching regulator 19. The switching regulator control circuit 15 turns on or off the switching transistor 13 so that the voltage V2 reaches a target value of 6 V for the intermediate voltage.
The output voltage or intermediate voltage of V2 from the switching regulator 19 is applied to emitters of the transistors 21 and 25 in the series regulators 23 and 27, respectively.
In the series regulator 23, the series regulator control circuit 22 monitors the collector voltage V3 for the transistor 21, namely the output voltage from the series regulator 23. The series regulator control circuit 22 continuously controls a base current for the transistor 21 so that the voltage V3 reaches a target value of 1.2 V for the operating voltage of the core 2.
The output voltage V3 from the series regulator 23 is output to the microcomputer 1 and is supplied as the operating voltage to the core 2 in the microcomputer 1.
In the series regulator 27, the series regulator control circuit 26 monitors the collector voltage V4 for the transistor 25, namely the output voltage from the series regulator 27. The series regulator control circuit 26 continuously controls a base current for the transistor 25 so that the voltage V4 reaches a target value of 5 V for the operating voltage of the I/O port 3.
The output voltage V4 from the series regulator 27 is output to the microcomputer 1 and is supplied as the operating voltage to the I/O port 3 in the microcomputer 1.
In the power supply apparatus 100, the reset control circuit 30 monitors the output voltage V4 from the series regulator 27. When detecting that the voltage V4 becomes lower than a specified voltage Vth as shown in FIG. 5A, the reset control circuit 30 outputs a active-low reset signal INIT to the microcomputer 1.
When the battery voltage instantaneously drops due to an electric load requiring a large amount of power, the power supply voltage V1 to the power supply apparatus 100 also drops instantaneously in the onboard ECU. The power supply voltage V1 is supplied through an ignition switch and a relay controlled in accordance with the ignition switch. When the ignition switch or the relay momentarily turns off, the power supply voltage V1 is also temporarily removed.
When the power supply voltage V1 drops suddenly due to an instantaneous interruption, the output voltage V2 from the switching regulator 19 also decreases as shown in FIG. 5A. The output voltages V3 and V4 from the series regulators 23 and 25 also decrease accordingly. The voltage V4 decreases first because the voltage V4 is originally set to be higher than the voltage V3.
As shown in FIG. 5A, the specified voltage Vth is so configured that the voltage V4 reaches the specified voltage Vth to reset the microcomputer 1 before the voltage V3 decreases to a minimum operating voltage Vmin for the core 2 due to a decrease in the voltage V2. The minimum operating voltage for the core is equivalent to a minimum operating voltage within a normal range.
Normally, the operating voltage V3 itself supplied to the core 2 should be monitored. However, the voltage V4 is monitored because the operating voltage V3 for the core 2 is low originally. For example, let us suppose that the core 2 uses an operating voltage in a normal range of 1.2 V ±10%. The construction of monitoring the voltage V3 needs to detect a decrease of a very small voltage such as approximately 0.1 V from 1.2 V, the center value of the normal range. It is difficult to implement a voltage detection circuit capable of detecting such a small voltage change.
As the microcomputer for the onboard power supply apparatus features high-speed operations, the core increasingly lowers an operating voltage and consumes more power.
The voltage monitoring method as shown in FIGS. 4 and 5A causes a degraded microcomputer reset response to an instantaneous drop of the power supply voltage V1 and cannot achieve secure fail safe.
Let us suppose that the core of the microcomputer 1 in FIG. 4 consumes a large current. FIG. 5B shows that the output voltage V2 from the switching regulator 19 decreases when the power supply voltage V1 drops due to an instantaneous interruption, for example. The voltage V3 decreases more steeply than voltage V4 because the core consumes a large current. The voltage V3 becomes lower than the minimum operating voltage Vmin for the core before the voltage V4 becomes lower than the specified voltage Vth, that is, before the microcomputer is reset. A microcomputer or core operation is unstable after the voltage V3 becomes lower than the minimum operating voltage Vmin for the core until the microcomputer is reset. Accordingly, data may be destroyed. An element included in the core may operate unstably. In the worst case, the microcomputer itself may be damaged. Also, the voltage V2 in FIG. 5B decreases more steeply than in FIG. 5A because the core consumes a large current.
One way to address the above described scenarios may be to increase the capacity of the capacitor C3 for voltage stabilization in FIG. 4 and gently decrease the voltage V3. However, in such an approach, the circuit scale and costs increase rendering the technique impractical.